The present invention relates to a mechanism for use with a sliding door of a railway car and more particularly relates to a mechanism for moving a sliding door of a railway car between a first position in which the bottom surface of the sliding door is supported on the body of the railway car and a second position in which the sliding door is supported by the mechanism and is moveable along the railway car body.
When a railway car is in use, cargo is loaded into the railway car, transported to another location and removed from the railway car. As is well known, a railway car body has at least one opening through which cargo may be loaded and unloaded. When in transit, it is necessary to close this opening in the railway car body. Accordingly, a sliding door is provided to close the opening when the railway car is in transit. When the railway car is in transit, the sliding door is sealed against the railway car body and the cargo retained therein. When the cargo reaches its destination, it is necessary to remove the sliding door from the opening in the railway car body so that the cargo could be removed therefrom.
The prior art has shown various mechanisms which allow for movement of the sliding door between a first position in which the sliding door is supported on the railway car body and closed the opening in the railway car body and a second position in which the sliding door is supported by the mechanism and is moveable along the railway car body to allow the cargo to be removed through the opening in the railway car body.
One such prior art mechanism is disclosed in U.S. Pat. No. 2,992,461 to Madland. Madland discloses a lifting and supporting mechanism for sliding doors of railway box cars having an operating lever rotatably mounted on the sliding door and operatively connected by means of interconnecting linkage to a roller housing. The roller housing has a main and an auxiliary roller rotatably supported thereon. The main roller is capable of moving between an extended and a retracted position. The main roller is moveable from the retracted position through an opening in the bottom of the door and into contact with a door track on the railway car body. In the retracted position, the main roller is inside the sliding door.
In Madland, to move the main roller to an extended position whereby it comes in contact with the track and subsequently raises the door from the track, the operating lever is actuated and through the interconnecting linkage moves a roller hanger to which the main roller is rotatably connected. The auxiliary roller is in contact with a bearing member having a surface which converges towards the track on the railway car body. The force exerted by the operating lever on the roller hanger moves the roller hanger along the bearing surface of the bearing member in a direction so that the main roller comes into contact with the track and subsequently lifts the sliding door off of the track. In this position, the sliding door is moveable along the track on the main rollers.
When Madland's operating handle is returned to its original position, springs are provided in the linkage to assure the return of the roller housing to a position in which the main rollers are retained inside of the sliding door so that the sliding door rests on the railway car body. It should be understood that the main rollers move through openings in the bottom of the sliding door and an upturned flange is provided. One side of the roller hanger is always in a lateral overlapping relationship with the upturned flange extending inwardly from the bottom of the sliding door, so as to prevent the rollers from wearing and gouging between the roller and door.
One of the inherent problems in the lifting and supporting mechanism disclosed by Madland is that the size of the main roller is restricted. This problem is inherent in the Madland design since there must be sufficient room so that the main and auxiliary rollers do not interfere while keeping the entire mechanism in a compact space. It is obvious that if a roller of substantial diameter is used, the roller friction is decreased and accordingly, a lesser force is required to move the railway car door along the track. Another problem with the mechanism disclosed by Madland is the substantial amount of force which must be applied to the roller hanger in order to lift the door off of the railway car body. It can be seen that the roller hanger must move downwardly a sufficient distance to move the roller from inside of the sliding door to the outside of the door and lift the door off of the track a sufficient distance to allow it to roll along the track. When a substantially horizontal force is exerted on the roller hanger, the bearing member exerts not only a vertical lifting component force but also a horizontal component opposite in direction to the horizontal force exerted on the roller bracket.
It should be noted that within the design constraints on the Madland design, the vertical distance through which the roller housing must be moved in order to move the roller a sufficient distance is limited. Thus, the angle of the bearing surface of the bearing member is substantial which in turn generates substantial opposing horizontal forces. The opposing forces are further increased when dirt, silt and other foreign matter are lodged on the bearing surface of the bearing member which increases the frictional resistance to movement of the roller hanger. Thus, a greater force must be exerted on the roller hanger to raise the door. In addition, the rolling friction on the auxiliary roller creates yet another frictional force which tends to oppose raising the sliding door from the track. Yet another problem associated with the mechanism disclosed by Madland is that it is extremely difficult to provide lubrication between the bearing surface and the auxiliary roller since the entire mechanism is enclosed in a housing.
Yet another problem associated with the mechanism disclosed by Madland is that once it is desirable to lower the sliding door onto the track, springs are necessary to assure the return of the roller housing back to a retracted position so that the main roller is again positioned inside of the railway car door. This spring is also necessary to assure that the main roller and roller housing does not move while the railway car is in transit and vibrate against the track and thus damage the mechanism. This complex linkage requiring the use of the spring requires substantial metal working and additional parts in order to properly secure the spring between the linkage and the sliding door.
Another known mechanism for lifting and supporting the sliding doors of railway cars is disclosed in Dietrichson, U.S. Pat. No. 2,682,075. Dietrichson discloses a mechanism for lifting and supporting sliding doors of railway cars which includes a lift lever pivotally supported on one end thereof to the sliding door. A roller is mounted on the lever adjacent to the pivotal connection. The roller is capable of moving through an opening in the bottom of the door. The roller may be moved through the opening a sufficient distance so that the door is lifted from the frame of the railway car and may be rolled therealong.
To move the lift lever, a handle is rotatably supported on the door and has arms extending therefrom. The arms are positioned adjacent to the bearing plates When the handle is rotated, the arms move into contact with the bearing plat to urge the lift lever in a direction so that the roller is moved through the opening in the bottom of the door and into contact with the railway car body. One of the problems with this mechanism is that due to the extreme length of the lift lever disclosed by Dietrichson, it is necessary for the arms of the handle to be of substantial length to move the lift lever a sufficient distance. Due to this substantial length of the arms, the mechanical advantage is decreased and a greater force must necessarily be exerted on the lift lever. Consequently, a greater force must be exerted on the handle.
Yet another problem inherent in the mechanism disclosed by Dietrichson is that the mechanical advantage is developed over the length of a long lift lever and not at a location close to the roller. This design characteristic of the mechanism disclosed by Dietrichson requires substantial forces to be transmitted by the lift lever.
As can be clearly seen from the above, the extreme length of the lift lever requires that substantial forces be transmitted through the lift lever in order to move the rollers and correspondingly lift the door. Accordingly, Dietrichson provides a plurality of guide plates to restrict the twisting, bending and buckling of the lift lever. It should be understood that the twisting and bending and misalignment of the lift lever creates a binding between the lift lever and the guide members. Accordingly, the lift lever may bind against the guide members which produces additional forces to be overcome by the operating handle.
Another problem with the mechanism disclosed by Dietrichson is that the interface between the bearing plate and the arm of the operating handle is in sliding frictional contact with each other when the mechanism is in operation. This sliding friction requires additional forces to be exerted on the operating handle. In addition, this interface betwen the bearing plate and the arm should be constantly lubricated to minimize the abovementioned frictional losses. Dietrichson does not provide any means for readily lubricating this interface since the entire mechanism is entirely enclosed in the side of the sliding door.
Yet another problem involved with the mechanism disclosed by Dietrichson is that when the operating handle is returned to its original or normal position, the rollers should be retracted within the door. Dietrichson does not provide any positive return means for returning the rollers through the slots in the bottom of the door so that they are positioned inside of the sliding door. Rather, Dietrichson relies on the weight of the door to return the rollers to a position inside the door. This disadvantageous feature is particularly important when there is ice or dirt underneath the door which prevents the rollers from returning to their normal position. In addition, when the railway car is in transit, the rollers are free to vibrate against other door components since the rollers are not positively returned or otherwise restrained when the sliding door is in the normally closed position.